In particular in internal combustion engines and transmissions in the automotive sector, particular requirements with respect to service life, low friction, and installation safety are placed on seal rings used there. A main direction of rotation is often present in such applications, i.e., the to-be-sealed shaft rotates predominantly in this main direction of rotation. A rotating in the opposite direction (so-called “reverse travel”) then only occurs in a very short time period. In this regard in the development to date of seal elements for these applications a focus has been placed on the achieving of a high and reliable tightness with rotating in the main direction of rotation and with stationary shaft, while the tightness with reverse travel was often secondary.
For example, radial shaft seal rings are known including polytetrafluoroethylene (PTFE) and a spiral pumping structure, for example, for a synthetic oil of the internal combustion engine. The sliding properties of PTFE-based shaft seals can be improved by additives such as graphite or molybdenum sulfide. However, PTFE here is relatively inelastic, with the result that disadvantages arise with respect to the static tightness of corresponding shaft seals. It is thus known to close an existing seal gap with waxes or greases for pressure- and/or vacuum-tests of such shaft seals. These volatilize in operation of the seal.
Improved seal assemblies have already been proposed that reduce such problems. Thus from DE 10 2007 036 625 A1 (a family member of US 2010/0237567) a seal element is proposed for sealing a shaft intended for rotating in a passage opening of a housing for the shaft, which seal element includes a stiffening part and an elastomer element connected thereto. The elastomer part comprises a first seal region for a static sealing abutment on the housing part, as well as a second seal region including a seal section configured and provided for sealing abutment on the shaft. The seal section includes a screw thread-type return pumping structure and a ring-type region, in the form of a closed line, lying on the free axial end. Due to the use of an elastomer the seal assembly has an increased elasticity, with the result that a pressure- and/or vacuum-testing is possible without applying additional materials onto the seal section. With the shaft rotating in the main direction, escaping leakage fluid is pumped by the return pumping structure toward the to-be-sealed space. In contrast, with a stationary shaft the ring-type region acts as a static seal.
Numerous proposed seal assemblies have the problem that a leakage results with a reversing of the shaft rotation direction. Solutions have also already been proposed for this problem. Thus in DE 10 2004 020 966 A1 (a family member of US 2005/0242521) a radial shaft seal is disclosed that is formed from an elastomeric material and includes a surface section that is provided with hydrodynamically acting return-pumping grooves for leakage liquid. These return-pumping grooves extend on the axial free end of the surface section in a seal bead. The return-pumping grooves are formed by inwardly directed cutting tips. For sufficient sealing the seal bead has a minimum contact width of 0.1 mm with the shaft. With the proposed seal assembly the contact width should be at most 0.8 mm, since otherwise a sufficient lubricating cannot be ensured. Due to this comparatively large contact width wear is increased.